More marketers are selling products via the e-commerce giant and need to run ads.

It's not clear where Amazon's ad budgets will come from long term.

It's constantly referred to as a "sleeping giant" in ad circles. WPP CEO Martin Sorrell says it keeps him up at night. And now, some ad buyers are seeing Amazon quickly emerge as a crucial ad vehicle, one that could someday have a serious impact on Google and Facebook.

"They are becoming a significant player," said Bryan Wiener, the executive chairman at the ad agency 360i.

As Wiener noted, more and more commerce-based searches are starting on Amazon. "They are starting to take revenue off of Google's plate. It's just started to reach a tipping point."

There's a simple explanation for that. After not doing so in the recent past, many of 360i's clients now sell their products on Amazon. And these marketers are finding that buying ads to spur sales on the site is critical. Thus, 360i is trying to quickly build out its Amazon ad expertise.

Michael Duda, a managing partner at the agency and investment firm Bullish, said his team recently recommended pulling ad spending from Facebook and Google to ramp up activity with Amazon — and saw sales jump by roughly 10%.

Silos are an obstacle

Amazon faces a unique challenge as it looks to draw larger marketing budgets. According to Wiener, for many brands Amazon ad spending ends up being managed by the same teams that handle things like in-store product placement and signage — and not the digital ad-buying teams that handle spending on Facebook and Google ads.

That makes it potentially harder, for example, to compare the performance of Amazon ads with other digital channels using the same sets of tools and analytics, or to make decisions about when and where to move money, Wiener said.

Among Amazon's other challenges: Some ad buyers say the company can be rigid in terms of how its ads can be purchased and how its data can and cannot be used outside its own properties. In other words, the same kind of "walled garden" complaints that get thrown toward Google and Facebook.

Plus, Amazon's strength right now seems to be in getting people to buy things they were already interested in and less about telling people about products they may never have heard of.